This invention relates to a process and apparatus for monitoring the quality of welds, in particular those produced by robot welding machines on an assembly line.
In the manufacture of large sheet metal structures, such as the bodies of automobiles and trucks, the standard procedure is to assemble several pre-shaped panels by means of a minimum number of initial welds and then to place these partially assembled structures in specific locations on movable supports that carry them along an assembly line. As each support moves along the assembly line, it stops at a series of precisely defined stations, and at each station, additional welds are made in the work piece where the designers have determined them to be necessary to hold the panels together with the required rigidity and strength. Manufacturing technique has advanced to the point that every part of the structure is so precisely located at each stop that the welds can be made by welding guns supported and positioned by robots. The robot at that station can be programmed to bring a welding gun into certain positions in space, and the parts of the work piece to be welded by that gun will be positioned there, ready for the gun to make a specific pattern of additional welds.
Both the initial welds and the additional ones are spot welds produced by placing electrodes against opposite surfaces of a stack of two or more metal panels and forcing a high current through the electrodes and the small area of the panels directly between their tips to melt the metal in that area and blend it into a single lump, called a nugget. Before the current is turned on, mechanical pressure is applied to the electrodes to cause them to clamp the panels firmly together to establish the required conductive path through the panels.
The parameters of each weld are specific and must be maintained constant for the corresponding weld in every replica of the structure being manufactured. One of the most important parameters is the magnitude of the welding current, which is typically in the range of about 5KA to about 15KA. It is well known that too low a current will produce unsatisfactory and even dangerously weak welds, but surprisingly, the actual welding current is not being measured on existing automotive assembly lines during the manufacturing process. Instead, the proper current for a given location on a newly designed work piece is determined during the design and manufacturing set-up phase by measuring the current as sample welds are made and selecting the value of current found to make welds of the proper quality. The measurement is done by means of sensing apparatus not suitable for use during the formation of every weld during the manufacturing process. The conditions necessary to cause that current to flow are then programmed into a weld timer panel associated with the robot that will make replicas of that weld on the assembly line. From then on, while replicas of that structure are being produced, sometimes in very large numbers, reliance is placed on holding constant the parameters that determine the current and on checking the finished welds by human observation and test on the welded work pieces during assembly. Unfortunately, some of those parameters change with wear and age, and human observation is fallible, so that weak or even non-existent welds sometimes get past the inspectors.
This is a very serious matter. If bad welds escape detection, the vehicle that is supposed to be held together by them will not be as strong as it should be. At best, the vehicle may wear out sooner than it should, and the manufacturer will get a reputation for bad quality. At worst, in an accident, the weakened structure may fail to protect its occupants from injury. There is a growing body of law to the effect that manufacturers who deliberately fail to take care to manufacture products so that they will be as safe as they can reasonably be expected to be will not only be subjected to severe civil penalties but may even be held criminally liable.
In accordance with Ohm's Law, the magnitude of any current is determined by the voltage driving that current and the impedance of the circuit through which the current flows. The impedance through which spot-welding current must flow is determined principally by the characteristics of the metal panels, the pressure exerted on them by the electrodes, the cross-sectional area of the electrodes, especially at the region of contact between each electrode and the panel contacted by it, and the resistance of the welding cable connecting the electrodes to the power supply. The ohmic value of the impedance through which the voltage must drive the welding current need not be measured; all that is required is that the voltage have the proper value to cause the required welding current to flow through the circuit.
In A. C. welding, the power is supplied through a transformer and a silicon-controlled rectifier circuit. The current does not necessarily flow continuously during the formation of each weld but only during controlled intervals of time. The duration and timing of these intervals are controlled by the weld timer that governs the operation of the SCR circuit and the welding gun. The weld timer begins each operation in response to a signal from the robot that supports and positions the welding gun, and the robot, in turn, is controlled to start its program of making a pattern of welds by a signal from a programmable logic computer (PLC) when a work piece has moved into position in that robot's station. As presently used, the PLC, which is basically a computer, does not receive any feedback of information from the welding circuit and, therefore, does not sense any change in the welding current, although the PLC can respond to input signals rapidly enough to sense such change.
One of the most common reasons for change in the welding current is that the cable that carries current from the control panel to the welding gun wears out. The cable has to be flexible to accommodate the movements of the robot, and such cables are made of a bundle of slender, flexible wires enclosed in an opaque, insulating sheath that carries water to cool the cable during operation. The magnetic fields produced along the cable by the welding currents interact with each other and cause the cable to jump each time a current pulse passes through it, and this jumping, plus the less sudden movement caused by operations of the robot, gradually break the wires in the cable and raise its resistance to the flow of current. Instead of having most of the impedance of the welding circuit concentrated in the path of the current through the panels, some of it will be distributed along the cable. It is immaterial where the impedance is located; the magnitude of current will be inversely proportional to the total impedance, and as the cable impedance increases, the current carried by it to the welding site will decrease. There is some tolerance in the magnitude of current required to make a satisfactory weld, and if the strands in the cable break one at a time, the welds produced by the gun fed by that cable will not suddenly change from good to bad but will do so progressively. However, the cables do sometimes break suddenly. Other factors, such as the pneumatic pressure that forces the electrodes and the panels together firmly enough to reduce the impedance to the proper value at that critical part of the welding circuit, can also go bad slowly or suddenly.
Sudden equipment failures that drop the welding current to zero cause the welds to be entirely missing, and missing welds are easier for inspectors on the assembly line to detect than are welds that are present but are just not quite as strong as they should be. In order to limit the number of slightly weak welds that can occur in a structure being assembled, it is common for the inspectors to apply a harsher test, not on every weld, but from time to time. In that type of test, the inspectors use a hammer and chisel to try to force apart the panels that should be welded together. If the panels separate too easily, the weld will be shown to have been too weak, and it must be re-welded by human welders in a repair section. Even if the weld is strong enough to make the effort to pry the panels apart unsuccessful, the structure may still have to be worked on to eliminate the damage to the metal caused by the destructive effort.
Nishiwaki et al. have proposed a sensing circuit in U.S. Pat. No. 4,739,149 to sense the magnitude of welding current by generating a voltage across a coil magnetically coupled to a cable carrying the welding current. A fraction of the secondary voltage thus generated is rectified and applied to a circuit having one or two light-emitting diodes. The amplitude of the induced voltage is non-linearly proportional to the amplitude of the welding current, and in a simple circuit, the brightness of the LED is proportional to this voltage. In a more complex circuit, the LED is prevented from turning on until the welding current reaches a certain value, and in a still more complex circuit, one LED is lighted if any welding current is flowing and a second LED is lighted only if the welding current exceeds a certain value. In any of the circuits, the person operating the welding apparatus has to watch the lamp or lamps carefully to determine whether a sufficient welding current is flowing in the cable. This would be very difficult to do during a real manufacturing operation.